1. Field of the Invention
This invention relates in general to subsea wellhead assemblies and in particular to a subsea wellhead assembly utilizing a concentric tubing hanger and in which installation is accomplished without requiring orientation of the tubing hanger or the tree.
2. Description of the Prior Art
In the offshore wells of the typed concerned herein, a subsea wellhead will be located at the sea floor. Two or more strings of well casing will be supported by casing hangers which land and seal within the wellhead housing. Subsequently, a string of production tubing is lowered through the casing to be used for producing fluids from the well. A tubing hanger located at the upper end of the string of tubing lands in the uppermost casing hanger. A production tree is then lowered onto the wellhead housing, the tree having various valves for controlling the produced fluids.
It is necessary for the tubing hanger to have a passage that leads to the annulus surrounding the tubing. In the most common type of tubing hanger, the annulus passage and the production passage extend through the tubing hanger parallel with but offset from the longitudinal axis of the tubing hanger. The production tree will have a production stinger for connecting its production bore with the production bore in the tubing hanger, and an annulus stinger for the stabbing into the annulus bore.
It is also common to have electrical connectors or penetrators that extend through the tubing hanger. The electrical connectors connect downhole instruments to the vessel at the surface for measuring pressure, temperature and the like. Normally, an electrical connector is located on the upper end of the tubing connector for mating with electrical connectors protruding downward from the lower end of the tree.
During installation, because of the offset production and annulus bores and the electrical penetrators, the tubing hanger will be oriented relative to the subsea wellhead housing. The tree also will be oriented so that the electrical connectors and the stingers will make up as the tree lands on the wellhead housing. In deep water, the orientation to the desired degree can be difficult.
In another type of tubing hanger, the production bore in the tubing hanger is coaxial with the longitudinal axis of the tubing hanger and the wellhead housing. In this type, known as a concentric tubing hanger, the production stinger from the tree needs no orientation. Also, the annulus passage in the tubing hanger may have a concentric valve sleeve, avoiding the need for orienting an annulus stinger of the tree with an annulus bore. However, if electrical penetrators are employed through the tubing hanger, orientation is still required for these connectors. In deep water, the orientation can still be a problem with concentric tubing hangers. Also, sealing for the annulus valves can be troublesome.